Byproduct streams commonly produced by secondary zinc operations (particularly those associated with recovering zinc from steel mill residuals and byproducts), secondary aluminum operations and aluminum dross processing operations typically have an appreciable salt content. This salt content can be problematic for the further processing and recovery of these materials. Traditionally, the majority of these high salt content materials have ended up in either solid or hazardous waste landfills. In recent years, it should be noted, more of these materials have been recycled, yet the salt fraction still has ended up in landfills or been deep well injected.
In the steel industry, two predominant streams that contain salt are electric arc furnace dust ("EAF Dust") and basic oxygen furnace dust ("BOF" dust). The U.S. Environmental Protection Agency has considered EAF Dust a hazardous waste when landfilled due to its potential to leach any of the four following toxic elements; cadmium, lead, nickel, and hexavalent chromium. BOF Dust is not considered a hazardous waste when landfilled as it typically contains only trace amounts of the previously listed toxic elements. The average EAF Dust has a total salt content of approximately 6%, and the average BOF Dust has a total salt content of approximately 2%.
The salt in these residual streams primarily originates as a coating on automobiles from roadways in the northern climates and/or the coastal areas where salt is picked up from the air. Salt is vaporized from shredded automobile scrap when charged to high temperature furnaces and ends up as a dust particulate captured by the offgas system. These salts combine with all the other particulate generated by the furnace operation and becomes EAF or BOF Dust.
Various processes have been devised to process EAF and BOF Dusts for the recovery of metals contained in the dusts as an alternative to landfilling. As a result of these processes, the salts are concentrated into a residual material stream that is sent to hazardous waste landfills.
Secondary aluminum processing operations and dross recovery plants use a mixture of potassium chloride and sodium chloride salts as a melting flux during furnace operations to maximize the recovery of aluminum metal. Aluminum has a high affinity for combining with oxygen. Furnace operators have found that a layer of molten salts upon the molten aluminum helps minimize the formation of aluminum oxide compounds, which minimizes the corresponding loss in metal yields during melting operations.
In secondary aluminum processing operations, molten salts combine with any aluminum oxides formed during the furnace operations forming aluminum dross. The aluminum dross is typically raked from the furnace into large pans which allows the aluminum dross to cool into large chunks or shapes. It is not uncommon for a small percentage of aluminum metal to become entrapped in the aluminum dross. Aluminum dross typically contains sufficient amounts of aluminum metal to economically support metal recovery activities. In fact, an entire industry has prospered through processing aluminum dross for the recovering of aluminum metal. However, aluminum dross processing operations also produce a byproduct, similar to dross, which is commonly referred to as "saltcake." This saltcake typically contains insufficient metal value to support recycling using conventional techniques. Although a small number of saltcake processors have prospered through competition with regionally expensive landfills and profitable sale of products recovered therefrom, saltcake is commonly disposed of in solid waste landfills.
The wet processing of zinc dross, aluminum dross and saltcake results in the production of brine. Most brine typically contains a number of different salts. Sodium chloride, potassium chloride, and magnesium chloride are the most common salts found in brine. Commercial salt-production-from-brine processes utilize the basic principle of exceeding the saturation level for each salt in the brine through evaporation of the water thereby causing the salts to selectively precipitate out of solution. The most common commercial technique for selective recovery of individual salts involves the use of a series of solar evaporation ponds. As salts in solution have different saturation limits, through the controlled movement of brine through a series of ponds during evaporation, the various salts can be forced to precipitate in separate ponds for subsequent recovery as individual products. It is not uncommon for mixtures of salts to be pre-precipitated in staging ponds as the brine pond manager strives to produce high purity products from this extremely large, and often unwieldy process. One common mixture of pre-precipitated salts is sodium chloride and potassium chloride that can be further processed to be ideally suited for use as a melting flux by the aluminum industry. Aluminum furnace operators have found that a mixture of sodium chloride and potassium chloride melts at a lower temperature than either salt alone, thus reducing energy costs.
A number of difficulties can arise when using conventional wet processing techniques to recover brine, metal, and metal oxides from, for example, aluminum dross or saltcake. Typically, blocks of aluminum dross or saltcake must be crushed into small pieces prior to subsequent processing. This can require equipment which is extremely capital intensive. Additionally, when water is used to leach the salts from the crushed mixture, the fine metallics react with the water resulting in a reduction of the metal recovery yield as well as the formation of hydrogen and ammonia gases. This evolution of gases makes it very difficult to separate the fine metallics and the aluminum-oxides from the brine in subsequent liquid-solid separation processes. For a detailed review of known wet processing techniques, reference is made to U.S. Pat. Nos. 5,013,356 to Olper et al., 5,290,535 to Zuck et al., and 5,330,618 to Daniels et al.
Accordingly, there is a need for an efficient and economical recycling method which can be operated at the site of the residual's generation that results in 100% recovery of residual byproducts to "close the loop" on various metallurgical processes. Such a method should maximize high value metal recovery and utilize waste heat from primary metallurgical operations for optimum cost efficiency.